Okay, I hadn’t seen it mentioned in the post and was hoping that was all it would take. I have played it before, and I can’t say I noticed any major issues like you’re describing. I run an AMD card though, so that may make the difference on Linux.
I wish I had some other ideas, but ideas I’ve found on ProtonDB were always my best option. I will say that I’ve noticed performance issues on some Unity games previously. Whether it’s from the engine or bad optimization I don’t know though.